Changing stereoscopic render output to "SIDEBYSIDE" with python
That can be set with bpy.context.scene.render.image_settings.stereo_3d_format.display_mode = 'SIDEBYSIDE' = 'SIDEBYSIDE'.

You can set other properties there too, like ...stereo_3d_format.use_sidebyside_crosseyed.

For more options, see the Stereo3dDisplay API documentation.